An ammeter is a device used for measuring the electrical current present in a circuit. A common type of ammeter is the current clamp ammeter, or as it is sometimes called the current probe ammeter. The current clamp ammeter is a device with two jaws that open and close to allow the jaws to be clamped around an electrical conductor. By closing the jaws, the electrical conductor is thereby circumscribed by the device.
A current clamp ammeter can measure various properties of the electrical current in the conductor. The most common type of current clamp ammeter is the current transformer, which measures current by treating the conductor within the clamp as the primary coil in a transformer, while a wire coil wound around one or both halves of the clamping jaws forms the secondary winding. These devices are particularly suitable for measuring alternating current (AC) passing through a conductor, such as is used in household and many industrial and commercial applications throughout the United States, Canada, and numerous other countries.
While current clamp ammeters are simple to use and relatively inexpensive, they are only useful with respect to standard two-conductor cables if the conductors can be physically separated. This is because the measurement of the clamp ammeter around a multiple-conductor cable will be the vector sum of the currents flowing in the conductors. In the case of a typical two-conductor cable carrying AC current to and from a load, the current flows between the two conductors will cancel out, and the clamp ammeter will display a reading of zero. For this reason, the jaws of the clamp ammeter must be placed around only one of the two conductors in order for an accurate current reading to be achieved.
U.S. Pat. No. 7,755,347 to Cullen et al. teaches a monitor for measuring the current (and other parameters) associated with a power cable. This patent explains the ineffectiveness associated with measurements of current around a two-conductor cable using a typical current transformer, as illustrated in FIG. 1. In order to make such a device functional, the patent teaches that a portion of the insulating cable cover must be cut away so that a single conductor can be withdrawn and placed through the toroidal core of the current transformer, as shown in FIG. 2. The invention described in the patent is directed to a more complex monitor apparatus that employs a plurality of ring-mounted magnetic and/or electric field sensors.
The Cullen et al. '347 patent specifically teaches that an electrician's portable ammeter (i.e., a clamp ammeter) may be used in connection with the arrangement illustrated in FIG. 2, and particularly in connection with the use of a standard power strip. The patent teaches, however, that such an arrangement is undesirable due to two disadvantages. First, one of the conductors must be exposed, which although not expressly stated, would be undesirable as a safety hazard. Second, the device creating the load to which the power cable is connected should be powered down during the addition of the current transformer as a safety precaution, thereby causing undesired system downtime.
The Cullen et al. '347 patent also discusses the use of a “break-out” box for purposes of measuring current flow. A break-out box is a connector inserted between an outlet and a power strip that allows for the measurement of current flow by exposing the individual conductors. Because the conductors are individually exposed, a clamp ammeter may be used to measure current flow by simply placing the jaws of the clamp ammeter around one of the conductors. The patent teaches, however, that such an arrangement is undesirable because either the load equipment must be powered down before the box is inserted—which results in system downtime—or the outer insulation on the conductors must be removed while the equipment is powered, which creates a potential safety risk. The patent also teaches that such an arrangement is undesirable because readings will change as equipment is added or removed from the power strip, thereby necessitating repeated measurements, which are complicated by the need to reinsert the break-out box each time a reading is needed.
A standard power strip (also known in some countries as a “power board”) is a series of electrical outlets that are arranged, usually linearly, along a housing that is connected to the end of a flexible cable. The cable is comprised of electrical wiring in an insulated sheath. By connecting each outlet to the wires of the cable, a power strip allows multiple electrical devices to be plugged in, while only one outlet (to which the power strip itself is plugged into by means of the flexible cable) is occupied. These devices are commonly applied to various household uses when a number of electrical devices are in use in close proximity to each other, such that the number of available electrical outlets is smaller than the number of electrical devices requiring power. Common examples include computer equipment and audio/visual equipment. Such power strips typically include a circuit breaker to limit the current that is drawn through them, in order to protect the power strip from overload. The circuit breaker of the power strip also serves to protect the outlet from overload, which could create a fire or electrical shock hazard if the electrical system's circuit breaker or fuse-based overload protection system fails to operate properly.
Although typical power strips do provide overload protection, they generally do not provide the user with any indication concerning the current that is actually passing through the power strip at any given time. The user thus has no direct means of gauging whether adding an additional piece of equipment to an available plug on the power strip, or substituting one piece of equipment for another, is likely to trip the circuit breaker on the power strip. This functionality would also be highly desirable to help prevent overload of the outlet circuit onto which the power strip is attached. Although current meters could be built into power strips, this alternative would make power strips prohibitively expensive for most users, since the user would effectively be purchasing another ammeter for every power strip that the user purchases.
What is desired then is a means of measuring current flow with respect to a power strip that overcomes all of the limitations and concerns associated with the prior art devices, while providing such a means in a simple, inexpensive package. It is further desired that such a device be usable with a standard clamp-type ammeter, such that no special equipment is required in order for a current reading to be taken.
References mentioned in this background section are not admitted to be prior art with respect to the present invention.